1GPIO Interfaces
2 3This provides an overview of GPIO access conventions on Linux.
4 5These calls use the gpio_* naming prefix. No other calls should use that
6prefix, or the related __gpio_* prefix.
7 8 9What is a GPIO?
10===============
11A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
12digital signal. They are provided from many kinds of chip, and are familiar
13to Linux developers working with embedded and custom hardware. Each GPIO
14represents a bit connected to a particular pin, or "ball" on Ball Grid Array
15(BGA) packages. Board schematics show which external hardware connects to
16which GPIOs. Drivers can be written generically, so that board setup code
17passes such pin configuration data to drivers.
18 19System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
20non-dedicated pin can be configured as a GPIO; and most chips have at least
21several dozen of them. Programmable logic devices (like FPGAs) can easily
22provide GPIOs; multifunction chips like power managers, and audio codecs
23often have a few such pins to help with pin scarcity on SOCs; and there are
24also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
25Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
26firmware knowing how they're used).
27 28The exact capabilities of GPIOs vary between systems. Common options:
29 30 - Output values are writable (high=1, low=0). Some chips also have
31 options about how that value is driven, so that for example only one
32 value might be driven ... supporting "wire-OR" and similar schemes
33 for the other value (notably, "open drain" signaling).
34 35 - Input values are likewise readable (1, 0). Some chips support readback
36 of pins configured as "output", which is very useful in such "wire-OR"
37 cases (to support bidirectional signaling). GPIO controllers may have
38 input de-glitch/debounce logic, sometimes with software controls.
39 40 - Inputs can often be used as IRQ signals, often edge triggered but
41 sometimes level triggered. Such IRQs may be configurable as system
42 wakeup events, to wake the system from a low power state.
43 44 - Usually a GPIO will be configurable as either input or output, as needed
45 by different product boards; single direction ones exist too.
46 47 - Most GPIOs can be accessed while holding spinlocks, but those accessed
48 through a serial bus normally can't. Some systems support both types.
49 50On a given board each GPIO is used for one specific purpose like monitoring
51MMC/SD card insertion/removal, detecting card writeprotect status, driving
52a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
53watchdog, sensing a switch, and so on.
54 55 56GPIO conventions
57================
58Note that this is called a "convention" because you don't need to do it this
59way, and it's no crime if you don't. There **are** cases where portability
60is not the main issue; GPIOs are often used for the kind of board-specific
61glue logic that may even change between board revisions, and can't ever be
62used on a board that's wired differently. Only least-common-denominator
63functionality can be very portable. Other features are platform-specific,
64and that can be critical for glue logic.
65 66Plus, this doesn't require any implementation framework, just an interface.
67One platform might implement it as simple inline functions accessing chip
68registers; another might implement it by delegating through abstractions
69used for several very different kinds of GPIO controller. (There is some
70optional code supporting such an implementation strategy, described later
71in this document, but drivers acting as clients to the GPIO interface must
72not care how it's implemented.)
73 74That said, if the convention is supported on their platform, drivers should
75use it when possible. Platforms must declare GENERIC_GPIO support in their
76Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't
77work without standard GPIO calls should have Kconfig entries which depend
78on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as
79optimized-away stubs, when drivers use the include file:
80 81 #include <linux/gpio.h>
82 83If you stick to this convention then it'll be easier for other developers to
84see what your code is doing, and help maintain it.
85 86Note that these operations include I/O barriers on platforms which need to
87use them; drivers don't need to add them explicitly.
88 89 90Identifying GPIOs
91-----------------
92GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
93reserves "negative" numbers for other purposes like marking signals as
94"not available on this board", or indicating faults. Code that doesn't
95touch the underlying hardware treats these integers as opaque cookies.
96 97Platforms define how they use those integers, and usually #define symbols
98for the GPIO lines so that board-specific setup code directly corresponds
99to the relevant schematics. In contrast, drivers should only use GPIO
100numbers passed to them from that setup code, using platform_data to hold
101board-specific pin configuration data (along with other board specific
102data they need). That avoids portability problems.
103 104So for example one platform uses numbers 32-159 for GPIOs; while another
105uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
106type of GPIO controller, and on one particular board 80-95 with an FPGA.
107The numbers need not be contiguous; either of those platforms could also
108use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
109 110If you want to initialize a structure with an invalid GPIO number, use
111some negative number (perhaps "-EINVAL"); that will never be valid. To
112test if such number from such a structure could reference a GPIO, you
113may use this predicate:
114 115 int gpio_is_valid(int number);
116 117A number that's not valid will be rejected by calls which may request
118or free GPIOs (see below). Other numbers may also be rejected; for
119example, a number might be valid but temporarily unused on a given board.
120 121Whether a platform supports multiple GPIO controllers is a platform-specific
122implementation issue, as are whether that support can leave "holes" in the space
123of GPIO numbers, and whether new controllers can be added at runtime. Such issues
124can affect things including whether adjacent GPIO numbers are both valid.
125 126Using GPIOs
127-----------
128The first thing a system should do with a GPIO is allocate it, using
129the gpio_request() call; see later.
130 131One of the next things to do with a GPIO, often in board setup code when
132setting up a platform_device using the GPIO, is mark its direction:
133 134 /* set as input or output, returning 0 or negative errno */
135 int gpio_direction_input(unsigned gpio);
136 int gpio_direction_output(unsigned gpio, int value);
137 138The return value is zero for success, else a negative errno. It should
139be checked, since the get/set calls don't have error returns and since
140misconfiguration is possible. You should normally issue these calls from
141a task context. However, for spinlock-safe GPIOs it's OK to use them
142before tasking is enabled, as part of early board setup.
143 144For output GPIOs, the value provided becomes the initial output value.
145This helps avoid signal glitching during system startup.
146 147For compatibility with legacy interfaces to GPIOs, setting the direction
148of a GPIO implicitly requests that GPIO (see below) if it has not been
149requested already. That compatibility is being removed from the optional
150gpiolib framework.
151 152Setting the direction can fail if the GPIO number is invalid, or when
153that particular GPIO can't be used in that mode. It's generally a bad
154idea to rely on boot firmware to have set the direction correctly, since
155it probably wasn't validated to do more than boot Linux. (Similarly,
156that board setup code probably needs to multiplex that pin as a GPIO,
157and configure pullups/pulldowns appropriately.)
158 159 160Spinlock-Safe GPIO access
161-------------------------
162Most GPIO controllers can be accessed with memory read/write instructions.
163Those don't need to sleep, and can safely be done from inside hard
164(nonthreaded) IRQ handlers and similar contexts.
165 166Use the following calls to access such GPIOs,
167for which gpio_cansleep() will always return false (see below):
168 169 /* GPIO INPUT: return zero or nonzero */
170 int gpio_get_value(unsigned gpio);
171 172 /* GPIO OUTPUT */
173 void gpio_set_value(unsigned gpio, int value);
174 175The values are boolean, zero for low, nonzero for high. When reading the
176value of an output pin, the value returned should be what's seen on the
177pin ... that won't always match the specified output value, because of
178issues including open-drain signaling and output latencies.
179 180The get/set calls have no error returns because "invalid GPIO" should have
181been reported earlier from gpio_direction_*(). However, note that not all
182platforms can read the value of output pins; those that can't should always
183return zero. Also, using these calls for GPIOs that can't safely be accessed
184without sleeping (see below) is an error.
185 186Platform-specific implementations are encouraged to optimize the two
187calls to access the GPIO value in cases where the GPIO number (and for
188output, value) are constant. It's normal for them to need only a couple
189of instructions in such cases (reading or writing a hardware register),
190and not to need spinlocks. Such optimized calls can make bitbanging
191applications a lot more efficient (in both space and time) than spending
192dozens of instructions on subroutine calls.
193 194 195GPIO access that may sleep
196--------------------------
197Some GPIO controllers must be accessed using message based busses like I2C
198or SPI. Commands to read or write those GPIO values require waiting to
199get to the head of a queue to transmit a command and get its response.
200This requires sleeping, which can't be done from inside IRQ handlers.
201 202Platforms that support this type of GPIO distinguish them from other GPIOs
203by returning nonzero from this call (which requires a valid GPIO number,
204which should have been previously allocated with gpio_request):
205 206 int gpio_cansleep(unsigned gpio);
207 208To access such GPIOs, a different set of accessors is defined:
209 210 /* GPIO INPUT: return zero or nonzero, might sleep */
211 int gpio_get_value_cansleep(unsigned gpio);
212 213 /* GPIO OUTPUT, might sleep */
214 void gpio_set_value_cansleep(unsigned gpio, int value);
215 216 217Accessing such GPIOs requires a context which may sleep, for example
218a threaded IRQ handler, and those accessors must be used instead of
219spinlock-safe accessors without the cansleep() name suffix.
220 221Other than the fact that these accessors might sleep, and will work
222on GPIOs that can't be accessed from hardIRQ handlers, these calls act
223the same as the spinlock-safe calls.
224 225 ** IN ADDITION ** calls to setup and configure such GPIOs must be made
226from contexts which may sleep, since they may need to access the GPIO
227controller chip too: (These setup calls are usually made from board
228setup or driver probe/teardown code, so this is an easy constraint.)
229 230 gpio_direction_input()
231 gpio_direction_output()
232 gpio_request()
233 234## gpio_request_one()
235## gpio_request_array()
236## gpio_free_array()
237 238 gpio_free()
239 gpio_set_debounce()
240 241 242 243Claiming and Releasing GPIOs
244----------------------------
245To help catch system configuration errors, two calls are defined.
246 247 /* request GPIO, returning 0 or negative errno.
248 * non-null labels may be useful for diagnostics.
249 */
250 int gpio_request(unsigned gpio, const char *label);
251 252 /* release previously-claimed GPIO */
253 void gpio_free(unsigned gpio);
254 255Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
256GPIOs that have already been claimed with that call. The return value of
257gpio_request() must be checked. You should normally issue these calls from
258a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
259before tasking is enabled, as part of early board setup.
260 261These calls serve two basic purposes. One is marking the signals which
262are actually in use as GPIOs, for better diagnostics; systems may have
263several hundred potential GPIOs, but often only a dozen are used on any
264given board. Another is to catch conflicts, identifying errors when
265(a) two or more drivers wrongly think they have exclusive use of that
266signal, or (b) something wrongly believes it's safe to remove drivers
267needed to manage a signal that's in active use. That is, requesting a
268GPIO can serve as a kind of lock.
269 270Some platforms may also use knowledge about what GPIOs are active for
271power management, such as by powering down unused chip sectors and, more
272easily, gating off unused clocks.
273 274For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
275be informed of their use; a gpiolib driver's .request() operation may call
276pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call
277pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
278to succeed concurrently with a pin or pingroup being "owned" by a device for
279pin multiplexing.
280 281Any programming of pin multiplexing hardware that is needed to route the
282GPIO signal to the appropriate pin should occur within a GPIO driver's
283.direction_input() or .direction_output() operations, and occur after any
284setup of an output GPIO's value. This allows a glitch-free migration from a
285pin's special function to GPIO. This is sometimes required when using a GPIO
286to implement a workaround on signals typically driven by a non-GPIO HW block.
287 288Some platforms allow some or all GPIO signals to be routed to different pins.
289Similarly, other aspects of the GPIO or pin may need to be configured, such as
290pullup/pulldown. Platform software should arrange that any such details are
291configured prior to gpio_request() being called for those GPIOs, e.g. using
292the pinctrl subsystem's mapping table, so that GPIO users need not be aware
293of these details.
294 295Also note that it's your responsibility to have stopped using a GPIO
296before you free it.
297 298Considering in most cases GPIOs are actually configured right after they
299are claimed, three additional calls are defined:
300 301 /* request a single GPIO, with initial configuration specified by
302 * 'flags', identical to gpio_request() wrt other arguments and
303 * return value
304 */
305 int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
306 307 /* request multiple GPIOs in a single call
308 */
309 int gpio_request_array(struct gpio *array, size_t num);
310 311 /* release multiple GPIOs in a single call
312 */
313 void gpio_free_array(struct gpio *array, size_t num);
314 315where 'flags' is currently defined to specify the following properties:
316 317 * GPIOF_DIR_IN - to configure direction as input
318 * GPIOF_DIR_OUT - to configure direction as output
319 320 * GPIOF_INIT_LOW - as output, set initial level to LOW
321 * GPIOF_INIT_HIGH - as output, set initial level to HIGH
322 * GPIOF_OPEN_DRAIN - gpio pin is open drain type.
323 * GPIOF_OPEN_SOURCE - gpio pin is open source type.
324 325 * GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction
326 * GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction
327 328since GPIOF_INIT_* are only valid when configured as output, so group valid
329combinations as:
330 331 * GPIOF_IN - configure as input
332 * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
333 * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
334 335When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
336open drain type. Such pins will not be driven to 1 in output mode. It is
337require to connect pull-up on such pins. By enabling this flag, gpio lib will
338make the direction to input when it is asked to set value of 1 in output mode
339to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
340 341When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
342open source type. Such pins will not be driven to 0 in output mode. It is
343require to connect pull-down on such pin. By enabling this flag, gpio lib will
344make the direction to input when it is asked to set value of 0 in output mode
345to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
346 347In the future, these flags can be extended to support more properties.
348 349Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
350introduced to encapsulate all three fields as:
351 352 struct gpio {
353 unsigned gpio;
354 unsigned long flags;
355 const char *label;
356 };
357 358A typical example of usage:
359 360 static struct gpio leds_gpios[] = {
361 { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
362 { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
363 { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
364 { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
365 { ... },
366 };
367 368 err = gpio_request_one(31, GPIOF_IN, "Reset Button");
369 if (err)
370 ...
371 372 err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
373 if (err)
374 ...
375 376 gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
377 378 379GPIOs mapped to IRQs
380--------------------
381GPIO numbers are unsigned integers; so are IRQ numbers. These make up
382two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
383map between them using calls like:
384 385 /* map GPIO numbers to IRQ numbers */
386 int gpio_to_irq(unsigned gpio);
387 388 /* map IRQ numbers to GPIO numbers (avoid using this) */
389 int irq_to_gpio(unsigned irq);
390 391Those return either the corresponding number in the other namespace, or
392else a negative errno code if the mapping can't be done. (For example,
393some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
394number that wasn't set up as an input using gpio_direction_input(), or
395to use an IRQ number that didn't originally come from gpio_to_irq().
396 397These two mapping calls are expected to cost on the order of a single
398addition or subtraction. They're not allowed to sleep.
399 400Non-error values returned from gpio_to_irq() can be passed to request_irq()
401or free_irq(). They will often be stored into IRQ resources for platform
402devices, by the board-specific initialization code. Note that IRQ trigger
403options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
404system wakeup capabilities.
405 406Non-error values returned from irq_to_gpio() would most commonly be used
407with gpio_get_value(), for example to initialize or update driver state
408when the IRQ is edge-triggered. Note that some platforms don't support
409this reverse mapping, so you should avoid using it.
410 411 412Emulating Open Drain Signals
413----------------------------
414Sometimes shared signals need to use "open drain" signaling, where only the
415low signal level is actually driven. (That term applies to CMOS transistors;
416"open collector" is used for TTL.) A pullup resistor causes the high signal
417level. This is sometimes called a "wire-AND"; or more practically, from the
418negative logic (low=true) perspective this is a "wire-OR".
419 420One common example of an open drain signal is a shared active-low IRQ line.
421Also, bidirectional data bus signals sometimes use open drain signals.
422 423Some GPIO controllers directly support open drain outputs; many don't. When
424you need open drain signaling but your hardware doesn't directly support it,
425there's a common idiom you can use to emulate it with any GPIO pin that can
426be used as either an input or an output:
427 428 LOW: gpio_direction_output(gpio, 0) ... this drives the signal
429 and overrides the pullup.
430 431 HIGH: gpio_direction_input(gpio) ... this turns off the output,
432 so the pullup (or some other device) controls the signal.
433 434If you are "driving" the signal high but gpio_get_value(gpio) reports a low
435value (after the appropriate rise time passes), you know some other component
436is driving the shared signal low. That's not necessarily an error. As one
437common example, that's how I2C clocks are stretched: a slave that needs a
438slower clock delays the rising edge of SCK, and the I2C master adjusts its
439signaling rate accordingly.
440 441 442What do these conventions omit?
443===============================
444One of the biggest things these conventions omit is pin multiplexing, since
445this is highly chip-specific and nonportable. One platform might not need
446explicit multiplexing; another might have just two options for use of any
447given pin; another might have eight options per pin; another might be able
448to route a given GPIO to any one of several pins. (Yes, those examples all
449come from systems that run Linux today.)
450 451Related to multiplexing is configuration and enabling of the pullups or
452pulldowns integrated on some platforms. Not all platforms support them,
453or support them in the same way; and any given board might use external
454pullups (or pulldowns) so that the on-chip ones should not be used.
455(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
456Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
457platform-specific issue, as are models like (not) having a one-to-one
458correspondence between configurable pins and GPIOs.
459 460There are other system-specific mechanisms that are not specified here,
461like the aforementioned options for input de-glitching and wire-OR output.
462Hardware may support reading or writing GPIOs in gangs, but that's usually
463configuration dependent: for GPIOs sharing the same bank. (GPIOs are
464commonly grouped in banks of 16 or 32, with a given SOC having several such
465banks.) Some systems can trigger IRQs from output GPIOs, or read values
466from pins not managed as GPIOs. Code relying on such mechanisms will
467necessarily be nonportable.
468 469Dynamic definition of GPIOs is not currently standard; for example, as
470a side effect of configuring an add-on board with some GPIO expanders.
471 472 473GPIO implementor's framework (OPTIONAL)
474=======================================
475As noted earlier, there is an optional implementation framework making it
476easier for platforms to support different kinds of GPIO controller using
477the same programming interface. This framework is called "gpiolib".
478 479As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
480will be found there. That will list all the controllers registered through
481this framework, and the state of the GPIOs currently in use.
482 483 484Controller Drivers: gpio_chip
485-----------------------------
486In this framework each GPIO controller is packaged as a "struct gpio_chip"
487with information common to each controller of that type:
488 489 - methods to establish GPIO direction
490 - methods used to access GPIO values
491 - flag saying whether calls to its methods may sleep
492 - optional debugfs dump method (showing extra state like pullup config)
493 - label for diagnostics
494 495There is also per-instance data, which may come from device.platform_data:
496the number of its first GPIO, and how many GPIOs it exposes.
497 498The code implementing a gpio_chip should support multiple instances of the
499controller, possibly using the driver model. That code will configure each
500gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
501rare; use gpiochip_remove() when it is unavoidable.
502 503Most often a gpio_chip is part of an instance-specific structure with state
504not exposed by the GPIO interfaces, such as addressing, power management,
505and more. Chips such as codecs will have complex non-GPIO state.
506 507Any debugfs dump method should normally ignore signals which haven't been
508requested as GPIOs. They can use gpiochip_is_requested(), which returns
509either NULL or the label associated with that GPIO when it was requested.
510 511 512Platform Support
513----------------
514To support this framework, a platform's Kconfig will "select" either
515ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
516and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
517three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
518 519It may also provide a custom value for ARCH_NR_GPIOS, so that it better
520reflects the number of GPIOs in actual use on that platform, without
521wasting static table space. (It should count both built-in/SoC GPIOs and
522also ones on GPIO expanders.
523 524ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
525into the kernel on that architecture.
526 527ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib code defaults to off and the user
528can enable it and build it into the kernel optionally.
529 530If neither of these options are selected, the platform does not support
531GPIOs through GPIO-lib and the code cannot be enabled by the user.
532 533Trivial implementations of those functions can directly use framework
534code, which always dispatches through the gpio_chip:
535 536 #define gpio_get_value __gpio_get_value
537 #define gpio_set_value __gpio_set_value
538 #define gpio_cansleep __gpio_cansleep
539 540Fancier implementations could instead define those as inline functions with
541logic optimizing access to specific SOC-based GPIOs. For example, if the
542referenced GPIO is the constant "12", getting or setting its value could
543cost as little as two or three instructions, never sleeping. When such an
544optimization is not possible those calls must delegate to the framework
545code, costing at least a few dozen instructions. For bitbanged I/O, such
546instruction savings can be significant.
547 548For SOCs, platform-specific code defines and registers gpio_chip instances
549for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
550match chip vendor documentation, and directly match board schematics. They
551may well start at zero and go up to a platform-specific limit. Such GPIOs
552are normally integrated into platform initialization to make them always be
553available, from arch_initcall() or earlier; they can often serve as IRQs.
554 555 556Board Support
557-------------
558For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
559function devices, FPGAs or CPLDs -- most often board-specific code handles
560registering controller devices and ensures that their drivers know what GPIO
561numbers to use with gpiochip_add(). Their numbers often start right after
562platform-specific GPIOs.
563 564For example, board setup code could create structures identifying the range
565of GPIOs that chip will expose, and passes them to each GPIO expander chip
566using platform_data. Then the chip driver's probe() routine could pass that
567data to gpiochip_add().
568 569Initialization order can be important. For example, when a device relies on
570an I2C-based GPIO, its probe() routine should only be called after that GPIO
571becomes available. That may mean the device should not be registered until
572calls for that GPIO can work. One way to address such dependencies is for
573such gpio_chip controllers to provide setup() and teardown() callbacks to
574board specific code; those board specific callbacks would register devices
575once all the necessary resources are available, and remove them later when
576the GPIO controller device becomes unavailable.
577 578 579Sysfs Interface for Userspace (OPTIONAL)
580========================================
581Platforms which use the "gpiolib" implementors framework may choose to
582configure a sysfs user interface to GPIOs. This is different from the
583debugfs interface, since it provides control over GPIO direction and
584value instead of just showing a gpio state summary. Plus, it could be
585present on production systems without debugging support.
586 587Given appropriate hardware documentation for the system, userspace could
588know for example that GPIO #23 controls the write protect line used to
589protect boot loader segments in flash memory. System upgrade procedures
590may need to temporarily remove that protection, first importing a GPIO,
591then changing its output state, then updating the code before re-enabling
592the write protection. In normal use, GPIO #23 would never be touched,
593and the kernel would have no need to know about it.
594 595Again depending on appropriate hardware documentation, on some systems
596userspace GPIO can be used to determine system configuration data that
597standard kernels won't know about. And for some tasks, simple userspace
598GPIO drivers could be all that the system really needs.
599 600Note that standard kernel drivers exist for common "LEDs and Buttons"
601GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those
602instead of talking directly to the GPIOs; they integrate with kernel
603frameworks better than your userspace code could.
604 605 606Paths in Sysfs
607--------------
608There are three kinds of entry in /sys/class/gpio:
609 610 - Control interfaces used to get userspace control over GPIOs;
611 612 - GPIOs themselves; and
613 614 - GPIO controllers ("gpio_chip" instances).
615 616That's in addition to standard files including the "device" symlink.
617 618The control interfaces are write-only:
619 620 /sys/class/gpio/
621 622 "export" ... Userspace may ask the kernel to export control of
623 a GPIO to userspace by writing its number to this file.
624 625 Example: "echo 19 > export" will create a "gpio19" node
626 for GPIO #19, if that's not requested by kernel code.
627 628 "unexport" ... Reverses the effect of exporting to userspace.
629 630 Example: "echo 19 > unexport" will remove a "gpio19"
631 node exported using the "export" file.
632 633GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
634and have the following read/write attributes:
635 636 /sys/class/gpio/gpioN/
637 638 "direction" ... reads as either "in" or "out". This value may
639 normally be written. Writing as "out" defaults to
640 initializing the value as low. To ensure glitch free
641 operation, values "low" and "high" may be written to
642 configure the GPIO as an output with that initial value.
643 644 Note that this attribute *will not exist* if the kernel
645 doesn't support changing the direction of a GPIO, or
646 it was exported by kernel code that didn't explicitly
647 allow userspace to reconfigure this GPIO's direction.
648 649 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
650 is configured as an output, this value may be written;
651 any nonzero value is treated as high.
652 653 If the pin can be configured as interrupt-generating interrupt
654 and if it has been configured to generate interrupts (see the
655 description of "edge"), you can poll(2) on that file and
656 poll(2) will return whenever the interrupt was triggered. If
657 you use poll(2), set the events POLLPRI and POLLERR. If you
658 use select(2), set the file descriptor in exceptfds. After
659 poll(2) returns, either lseek(2) to the beginning of the sysfs
660 file and read the new value or close the file and re-open it
661 to read the value.
662 663 "edge" ... reads as either "none", "rising", "falling", or
664 "both". Write these strings to select the signal edge(s)
665 that will make poll(2) on the "value" file return.
666 667 This file exists only if the pin can be configured as an
668 interrupt generating input pin.
669 670 "active_low" ... reads as either 0 (false) or 1 (true). Write
671 any nonzero value to invert the value attribute both
672 for reading and writing. Existing and subsequent
673 poll(2) support configuration via the edge attribute
674 for "rising" and "falling" edges will follow this
675 setting.
676 677GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
678controller implementing GPIOs starting at #42) and have the following
679read-only attributes:
680 681 /sys/class/gpio/gpiochipN/
682 683 "base" ... same as N, the first GPIO managed by this chip
684 685 "label" ... provided for diagnostics (not always unique)
686 687 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
688 689Board documentation should in most cases cover what GPIOs are used for
690what purposes. However, those numbers are not always stable; GPIOs on
691a daughtercard might be different depending on the base board being used,
692or other cards in the stack. In such cases, you may need to use the
693gpiochip nodes (possibly in conjunction with schematics) to determine
694the correct GPIO number to use for a given signal.
695 696 697Exporting from Kernel code
698--------------------------
699Kernel code can explicitly manage exports of GPIOs which have already been
700requested using gpio_request():
701 702 /* export the GPIO to userspace */
703 int gpio_export(unsigned gpio, bool direction_may_change);
704 705 /* reverse gpio_export() */
706 void gpio_unexport();
707 708 /* create a sysfs link to an exported GPIO node */
709 int gpio_export_link(struct device *dev, const char *name,
710 unsigned gpio)
711 712 /* change the polarity of a GPIO node in sysfs */
713 int gpio_sysfs_set_active_low(unsigned gpio, int value);
714 715After a kernel driver requests a GPIO, it may only be made available in
716the sysfs interface by gpio_export(). The driver can control whether the
717signal direction may change. This helps drivers prevent userspace code
718from accidentally clobbering important system state.
719 720This explicit exporting can help with debugging (by making some kinds
721of experiments easier), or can provide an always-there interface that's
722suitable for documenting as part of a board support package.
723 724After the GPIO has been exported, gpio_export_link() allows creating
725symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
726use this to provide the interface under their own device in sysfs with
727a descriptive name.
728 729Drivers can use gpio_sysfs_set_active_low() to hide GPIO line polarity
730differences between boards from user space. This only affects the
731sysfs interface. Polarity change can be done both before and after
732gpio_export(), and previously enabled poll(2) support for either
733rising or falling edge will be reconfigured to follow this setting.
734